The present invention relates to communication systems. In particular, the present invention relates to an adaptable wideband wireless communications architecture incorporating code division and time division multiple access techniques.
Multiple access communications systems provide ordered access to a common communication channel to multiple users or nodes. Wireless communication systems are typically multiple access systems. As an example, cellular phone networks share one or more subbands in the radio frequency (RF) spectrum divided into time division multiple access (TDMA) time slots for multiple nodes. As another example, satellite receivers often receive a TDMA uplink centered in a very high frequency band (e.g., tens of gigahertz).
Communication systems are further characterized by duplex (sometimes referred to as full duplex) or simplex (sometimes referred to as half-duplex) operation. Simplex operation proceeds by allowing only one node to transmit over a connection while the other nodes are receiving on that connection. On the other hand, duplex operation allows simultaneous transmission and reception by the nodes on the connection.
In addition to time division techniques that provide access for multiple users, communication systems may use frequency division techniques. As an example, the original cellular telephone standard, AMPS (Advanced Mobile Phone Service), implements frequency division multiple access and frequency division duplex operation. In AMPS, the RF spectrum is divided into two broad bands, one for “forward” (base station to mobile) connections, and the other for “reverse” (mobile to base station) connections. Each band is further divided into multiple frequency channels, and each frequency channel is partitioned according to TDMA techniques to provide an overall capacity for thousands of nodes.
Frequency division techniques, however, have typically required interference limiting isolation between frequency bands. For example, past systems relied on complex and expensive radio frequency isolation equipment to maintain separation in the received signals. Furthermore, a significant amount of the RF spectrum was wasted on guard bands to further increase frequency isolation.
In recent years, a third type of communication technique, code division multiple access (CDMA), has started to emerge in commercial systems. In a CDMA system, an input data stream is modulated by a spreading code at a much higher data rate (sometime referred to as the chip rate) than that of the input data stream itself. The output of the modulation thus has many more transitions that the input data itself and eventually results in a transmitted signal “spread” over a wide frequency band.
The input data may be recovered at a receiver by correlating the received signal with the original spreading code. Other signals may be present in the received signal, including interference and additional spread spectrum signals (created with additional, uncorrelated spreading codes). However, in general, the additional signals produce an output that appears as random noise with respect to the desired spreading code. Thus, multiple nodes may transmit overlapping spread spectrum signals without preventing recovery of the other simultaneously transmitted signals.
Wireless communication systems typically reuse the frequency spectrum in physically separate locations to make the most of the RF resource. Thus, a reuse plan including multiple cells is established and may provide, for example, a seven-to-one reuse ratio. In other words, seven frequency bands are used and reassigned across the reuse plan to minimize co-channel interference and increase capacity.
Extensive planning must be performed beforehand to select, establish, and allocate cell sites and portions of the RF resource to reliably operate a wireless communication system. This process is made even more complicated by the fact that, in the past, fixed base station nodes were used in each cell site, and the base station nodes were interconnected with a hardwired network. Furthermore, past cell sites required very tightly controlled geometry in which, for example, no base station was allowed to deviate from cell alignment by more than 10%.
Using mobile base stations or deploying base stations without precise pre-planning for a wireless communication system was extremely difficult, and nearly impossible if the communication system were based on wireless trunks. One significant source of difficulty lay in the past use of duplex operation on separate frequencies. Such duplex operation invariably leads to a map problem as the network grows in which a node is both transmitted and receiving simultaneously on the same frequency, an extremely difficult task.
Thus, in the past, the pre-planning required to implement a communication system prevented the communication system from easily employing mobile base station nodes, or wireless trunks. In addition, past communication systems were generally unable to integrate new nodes into a network without the extensive pre-planning. As a result, past wireless communication networks have been static, immutable entities. In other words, it has not been possible to dynamically deploy, extend, and geometrically alter a wireless communication network without requiring expensive, time consuming, and complex transmission engineering for satisfactory operation. These shortcomings are even more prevalent with respect to wireless (as opposed to hardwired) trunks between nodes such as base stations. Thus, past wireless communication networks have been unduly limited in their size, bandwidth, and adaptability.
A need has long existed in the industry for a deployable, extendable, alterable communications network that does not incur the costs associated with complex transmission reengineering.